360 Commando head dilemma

Neither head will properly support a 408 so to me it's a wash. If the done heads have a flow test sheet with them they may be better than what you have. But stil not enough for a 408. A decently done set of ported 308s will work ok on a 4" engine and better than similar prepped set fo Xs or Js. But that's "just barely good enough". Not "good". If it was me I'd tell you to stick with what you have and continue saving for the right parts.